Peel and slice the bananas. Season well with curry powder. Fry in hot oil, then mash. Clean the leek, cut it into finger-thick slices and add it with the water and the stock. Refine with a pinch of rosemary. Let simmer for 10 minutes, then serve - the leek can still have a bit of bite.
Tip: If you like it a bit hotter, you can add two to four finely chopped Thai chili peppers.